Comparison of the effects of pancuronium and tubocurarine on different muscles of young and old mice.

In order to evaluate the sensitivity of different muscle types to neuromuscular blocking drugs, a system using mouse muscles in vitro was developed and applied to detect changes in drug sensitivity in relation to age. The effect of pancuronium and tubocurarine on initial twitch and on the ratio of fourth twitch to first twitch (T4/T1) of a train-of-four at 2 Hz were compared in fast-twitch, slow-twitch and respiratory muscles in the mouse. The muscles used were: extensor digitorum longus (EDL), soleus (SOL) and diaphragm (DIA). For both drugs the order of decreasing sensitivity was EDL greater than SOL greater than DIA. This result was the same whether first twitch or T4/T1 was used, although the latter was a more sensitive indicator. The sensitivity of neuromuscular block was less in muscles from old (30-33 month) animals than in the equivalent muscles from young (8-12 month) animals.